1. Field of the Invention
The embodiments discussed herein relate to an apparatus and a method for controlling a switching power supply.
2. Background of the Related Art
Many switching power supplies use a flyback topology for the alternate current (AC)/direct current (DC) converter.
Such a flyback switching power supply includes a transformer and a switching transistor such as a metal oxide semiconductor field effect transistor (MOSFET), causes a diode bridge to rectify an AC input voltage, and obtains a desired DC output voltage from the rectified voltage.
In addition, such an AC/DC converter is provided with a control circuit to supply a stable output voltage even when, for example, the AC input voltage, the load, or the temperature changes.
The control circuit is arranged on the primary side of the transformer. On the basis of information about the output voltage on the secondary side of the transformer, the control circuit performs feedback control on the switching of the MOSFET and maintains the output voltage at a constant level.
For the transmission of this output voltage information from the secondary side to the primary side, conventional flyback switching power supplies use an insulating element such as an output voltage error amplifier (a shunt regulator) or a photocoupler.
However, to reduce the cost, the number of parts, etc., power supplies that do not use such an output error amplifier or photocoupler have recently been developed. These power supplies perform feedback control on the output voltage by using a voltage that occurs across an auxiliary winding of the transformer and are referred to as primary-side-control flyback power supplies.
For example, according to a conventional technique for such a primary-side-control flyback power supply, the output voltage is controlled by detecting the primary voltage and correcting the voltage lost from the secondary voltage when the secondary current reaches zero, for example (Japanese National Publication of International Patent Application No. 2010-521954).
According to another conventional technique, a voltage that occurs across an auxiliary winding is detected and compared with a reference value, and a detection period is determined based on the comparison result. In addition, the detected voltage is sampled by using two pulses within the detection period, and one of the detected voltages is outputted (Japanese Laid-open Patent Publication No. 2013-121214).
A flyback power supply performs a control operation so that the output voltage has a desired value. A primary-side-control flyback power supply also includes a transformer having an auxiliary winding. Ideally, the primary-side-control flyback power supply obtains a voltage that corresponds to the output voltage from the auxiliary winding and controls the output voltage on the basis of the obtained voltage. More specifically, for example, the secondary winding and the auxiliary winding of the transformer are formed to have the same number of turns. A voltage that corresponds to the output voltage is generated across the auxiliary winding, and the voltage that occurs across the auxiliary winding in a secondary conduction period is detected. In addition, the output voltage is controlled by using a pulse width modulation (PWM) control circuit.
However, in reality, since a voltage drop is caused by a diode and the like arranged on the secondary side, the output voltage is changed from its target value, and an error is caused.
Thus, a control circuit for a simple primary-side-control flyback power supply does not perform the above error correction and is used when a high degree of accuracy is not needed. Meanwhile, means for correcting this error has also been proposed. For example, Japanese National Publication of International Patent Application No. 2010-521954 discusses sampling an auxiliary winding voltage a plurality of times in a secondary conduction period and performing a control operation by using a sampling result obtained immediately before the secondary current reaches zero and the auxiliary winding voltage begins to decrease. As another example, as discussed in U.S. Patent Application Publication No. 2010/0246216, there is known a technique in which the slope of an auxiliary winding voltage over time is monitored. According to this technique, a control operation is performed by using the auxiliary winding voltage when the secondary voltage reaches zero and the slope of the auxiliary winding voltage significantly changes. According to any of these techniques, the secondary current is determined to have reached zero by detecting when the auxiliary voltage begins to sharply decrease, and a control operation is performed by using an auxiliary winding voltage immediately before this timing.
In an operation in a discontinuous current mode (DCM) in which the secondary current reaches zero per switching period, these conventional techniques achieve accurate output control while eliminating the voltage lost on the secondary side. However, when the input voltage is low or the load current is large, an operation in a continuous current mode (CCM) could occur. In such a case, the secondary current has not reached zero when the auxiliary winding voltage begins to sharply decrease. Thus, according to the above conventional techniques, the error caused by the loss on the secondary side cannot completely be eliminated.